Method and apparatus for positioning a member relative to an object surface

ABSTRACT

A method and apparatus for positioning a member in a desired position relative to a surface of an object including providing positioning means for positioning the member, sensing the attitude of the object surface relative to the member or the positioning means using a non-contact sensing means, the attitude being sensed in at least one plane normal to the object surface, and controlling the positioning means using data from the sensing means to position the member at a desired attitude relative to the surface of the object.

This application is a division of application Ser. No. 08/334,350, filedNov. 2, 1994, now U.S. Pat. No. 5,510,625, which is a division ofapplication Ser. No. 08/124,605, filed Sep. 21, 1993, now U.S. Pat. No.5,362,970, which was a division of application Ser. No. 07/836,508,filed Feb. 18, 1992, now U.S. Pat. No. 5,280,170, which was a divisionof application Ser. No. 07/711,397, filed Jun. 6, 1991, now U.S. Pat.No. 5,164,579 which was a continuation of application Ser. No.07/511,967, filed Apr. 17, 1990, now abandoned, which was a continuationof application Ser. No. 07/381,032, filed Jul. 19, 1989, now abandoned,which was a continuation of application Ser. No. 07/262,131, filed Oct.25, 1988, now abandoned, which was a continuation of application Ser.No. 07/059,632, filed Jun. 8, 1987, now abandoned, which was acontinuation of application Ser. No. 06/757,208, filed Jul. 22, 1985,now U.S. Pat. No. 4,674,869, which was a continuation of applicationSer. No. 06/697,683, filed Feb. 1, 1985, now abandoned, which was acontinuation of application Ser. No. 06/634,191, filed Jul. 24, 1984,now abandoned, which was a continuation of application Ser. No.06/378,808, filed May 17, 1982, now abandoned, which was a division ofapplication Ser. No. 06/034,278, filed Apr. 30, 1979, now U.S. Pat. No.4,373,804.

FIELD OF THE INVENTION

This invention discloses method and apparatus for optically determiningthe dimension, location and attitude of part surfaces. Particularembodiments describe optical triangulation based co-ordinate measurementmachines capable of accurate measurement of complex surfaces, such asgear teeth and turbine blades.

The invention also discloses means for accurately sensing in up to 5axes the xy location, range and attitude in two planes of an object orobject feature, as well as the measurement of angular and size variableson the object.

There are many triangulation sensors in the present art, but none isknown to have achieved what this one discloses, namely 0.0001" or even50 millionths of an inch accuracy on complex surfaces, in a mannercapable of high speed analysis of surface form.

To be truly accurate on surfaces of widely varying form, the sensor musthave an ability to operate with huge (e.g. 10⁴) variations in returnedlight intensity from the part, without giving an apparent change indimension. And too, the sensor must provide high resolution, drift freedigital readout of part dimension. These and other features are presentin the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagrammatic view of an embodiment of the invention;

FIG. 2 is a block diagram of the basic elements of a system for movingthe rotary tale of FIG. 1;

FIG. 3 is a diagrammatic view of a further embodiment of the invention;

FIGS. 3A and 3B are electrical schematic diagrams of printed circuitsuseful in the embodiment of FIG. 3;

FIG. 4 is a graphical representation of a video output signal generatedby the diode array of FIG. 3;

FIGS. 5 and 6 are graphical representations of a scan of a diode arrayillustrating a technique for finding the optical center of a spot oflight which falls on the array;

FIG. 7 is a diagrammatic view of a further embodiment of the inventionparticularly adapted to the measurement of the pitch circle runout ofgears;

FIG. 8 is a diagrammatic view of a portion of the embodiment of FIG. 7;

FIG. 9 is a graphical are presentation of data generated by theembodiment of FIG. 7;

FIG. 10 is a diagrammatic view of a portion of a further embodiment ofthe invention adapted to the measurement of gear involute or helixangle;

FIG. 11 is a diagrammatic view of a portion of a further embodiment ofthe invention adapted to the measurement of the thickness of a smallflat pump vane;

FIG. 12 is a block diagram of an electronic system for performing themeasuring function in the embodiment of FIG. 11;

FIG. 13 is a graphical representation of a system in accordance with theinvention wherein a line image, as opposed to a spot image, is projectedonto the object under examination;

FIGS. 14 and 15 are diagrammatic views of further embodiments of theinvention adapted to sense yaw and pitch;

FIGS. 16 and 16A are diagrammatic views of a further embodiment ofinvention having capability of five axis sensing; and

FIGS. 17 and 18 are block diagrams of electronic systems particularlyuseful for sensing moving parts in accordance with the invention.

FIG. 1 illustrates apparatus useful for the contouring of threedimensional objects (for example, turbine blades and gears). This systemcan be operated in two distinct modes. In one of these modes, thetheoretical part shape is stored in a computer which moves the sensorunder its control through the theoretical shape of the part and anysignal from sensor indicates a part deviation from nominal size. Asecond mode is useful in scanning part surfaces to digitize the surfaceshape. In this second mode the sensor itself is used as a null seekingdevice and any movement of the sensor necessary to keep the systemnulled is recorded by the computer as the part shape.

One primary advantage of this type of system is that it isnon-contacting. The optical means of measurement allows one to measureparts that are either too soft to be touched by mechanical probes, ortoo brittle or fragile. In addition, even though the sensor itself mayhave a very small measurement range in the order of several tenths of aninch, the overall system can have measurement ranges in the order of 6to 10 inches because of the ability of high resolution precision tablesto move the sensor over these large areas. Also, there are no contactpoints to wear, or get caught in holes, and the system can also checkthe presence of holes. Thus there is provided a very accurate, localsensor reference that does not touch the part and additionally a highlyaccurate frame of reference capable of containing a measurement volumeof several cubic feet.

The system consists of three basic parts: a sensor, a moveable tablesystem; and a control computer.

The sensor (8) consists of an illuminating light source (1), in thiscase a laser, whose power output is controlled by a pockels cell (2).The light energy after passing through the pockels cell is focused to aspot 2A on the surface of the object (4) being measured, via a focussinglens (3). The light scattered from the object is picked up by an imaginglens (5) at angle δ and imaged onto a linear photo diode array (7). Thefunction of the mirror (6) is to reduce the overall package size. Thelinear photo diode array then is used to sense the position of theimaged light spot. For optimum accuracy δ should be in the range30°-50°. Current systems operate at 45°.

The optical system is aligned so that the image light spot is at thecenter of the linear photo diode array (7) when the surface of theobject is in its nominal position. As the object (4) is moved eithertowards or away from the sensor (8) the imaged light spot moves acrossthe photo diode array (7). The movement of the object (4) and themovement of the imaged light spot in the linear photo diode array (7)are related by similar triangles centered about the imaging lens (5).This technique is referred to as optical triangulation.

It is sometimes desirable, depending on the shape of the object, to havea duplicate light receiving system on the opposite side of theillumination spot. This would then consist of an additional imaging lens(12), an additional mirror (13) and an additional photo diode array(14).

It is important to note that it is desirable for highest accuracy tokeep the laser spot or light spot very small on the surface of theobject. A spot in the order of two to five thousandths of an inch ismost generally desirable.

This is desirable for two reasons. First, any surface reflectivityvariations across the light spot will result in centroid determinationerrors at the array. It has been observed that keeping the spot smallreduces these types of errors dramatically. Secondly, since the spot onthe object surface is magnified by the lens on the array, a small spoton the surface is needed to keep a reasonably small spot on the array.The small spot also allows tight curves to be gaged accurately.

In the case where a laser is used as the illumination source, there is,of course, the problem of speckle in the resultant image due to thecoherence of the illuminating light. The means used to avoid thisproblem are, first to use as large an aperture imaging lens as possible,and second to use a wide photo diode array. This wide array will tend toaverage in a transverse direction the speckle in the laser spot. Toillustrate, system accuracy was notably improved when, at amagnification of 3:1, a 0.017" wide diode array was used instead of the0.001" wide version previously employed.

A very important design consideration in the sensor is the amount oflight that is reflected from the object surface. In the normal course ofevents, an object surface can vary from very dull to very bright andfrom very rough to very smooth. These variations do modify the amount oflight that is reflected from the surface of the object and, therefore,the resultant light on the photo diode array. Since a certain minimumamount of light is necessary for detection of the spot, and since thepresence of too much light causes uncertainty in the location of thecenter of the spot, it is necessary to control the amount of lightfalling on the array or correspondingly, to control the integration timeof the array to allow more light gathering time. For this reason, amicro computer (19) is used to control both the voltage on the pockelscell which, therefore, controls the amount of light striking the surfaceof the object, and also to control the operating speed or integrationtime of the linear photo diode array. Under normal operating mode, it isdesirable to keep the photo diode array running with as high a speed aspossible in order to maximize the speed of operation. Therefore, theprimary control mode is to vary the power arriving at the surface bysampling the array and, in a feedback loop, via the micro computer, tocontrol the photo diode array speed. If such a point is reached wherethe pockels cell is allowing the maximum amount of light from the laserto pass through it, then it is necessary to slow down the speed ofoperation of the photo diode array in order to work with surfaces thatare even duller than those with which it is possible to word bycontrolling only the laser power. In this case, the overall speed of thegage is reduced. However, it is still possible to make measurements fromdull surfaces. It is possible via this technique to work with surfaceswhose reflectivity varies by up to 5 orders of magnitude (10⁵ ×).

Another important point in the design of the sensor is the magnificationused in the system. It is desirable to have an optical magnificationranging between three times and five times in order to take the greatestadvantage of the spot movement in relation to the diode array size. Inaddition, it is possible to tilt the photo diode array at an angle tothe direction of normal light incidence. This serves two functions. Ithelps maintain the focus of the system over a large range of movementand, it also results in a higher effective magnification.

The moving table system allows the sensor head to have effectivelylarger range. For example, if the x-y tables (9) have a movement rangeof 6 inches each and resolution of 50 millionths of an inch, this, inconjunction with the sensor resolution being extremely high, allows oneto have a very large high accuracy measuring volume. It is also possiblefor the z axis table (Al) to have a movement of 12 to 14 inches. This,coupled with the ability to rotate the part a full 360°, allows one tofully contour a part by moving in turn the x, the y, the z or the 0axis. The control computer allows the design data for an object storedin its memory to be converted into movements of the sensor via the tablesystem. The basic elements of the system are shown in FIG. 2 where thereis shown a control computer (15), an input terminal (16), which is usedto enter commands as well as additional data into computer, an x-yplotter (17), which is used to plot out contours or deviations fromdesired contour, a 4 axis table controller (18), which accepts commandsfrom the control computer and thereby moves the sensor head to variouslocations in space and the sensor control micro computer (19), whichcontrols the scanning speed of the arrays as well as the voltage appliedto the pockels cell for light control. It also accepts information fromthe arrays or arrays (20) and (21), which will allow the micro computerto vary the array integration time.

Conversely, polar coordinate based systems can be used when the sensoris moved in r and θ, and possibly φ as well. Polar arrangements areadvantageous when contours of concave surfaces, for example, aredesired. Such a system is also of interest if rapidly varying curvaturesare encountered. Another advantage of a polar system is that the angularscan (θ or φ) can be mechanically very fast and smooth as it is providedby a high grade bearing motor and shaft encoder combination.

A further embodiment of the invention is shown in FIG. 3. Theelectro-optical system within the optical head consists of the laser(33), pockets cells (32), analyzer (34), beam focussing lens (31), partbeing measured (25), imaging lenses (26) (35), linear diode arrays (29)(38), and diode array control cards (30) (39). Printed circuit diagramsPC 7807 (FIG. 3A) and PC 7808 (FIG. 3B) are incorporated by reference.

The video signal produced by the diode array is sampled and held betweenclock pulses to produce a boxcar type video output similar to FIG. 4.The array control boards work with external clock and start signalssupplied by the light level compensation board P.C. 7808 (40). The videosignals are fed to the programmable filters (low pass) (41) and the fourvideo filter outputs per side are fed to their respective multi-levelspot centroid finder (42) where one of the signals is selected. Thevideo signal is then level detected, the resulting count pulse sequencesare counted and displayed for visual indication, and also sent to theA/D and Data Distribution card PC 7903 (43) where they are counted inbinary. The resulting binary number is then sent to the micro computer(44) where the data is scaled and linearized.

In operation, the external control computer signals the internalmicro-computer to initiate a reading. The microcomputer in turn signalsthe pockels cell controller (45) to take a light power reading. Oncompletion of this, the pockels cell is set by the controller to acertain illumination level. After this operation, the light levelcompensation circuit is signaled to initiate a sequence of diode arrayscans.

Once the left and optional right channel readings are obtained the datais sent to the micro-computer for processing, after which data is put onthe computer bus.

The linear diode arrays are set to operate from an external clock andeach scan is initiated by an external start command. The image of thelaser spot formed on the diode array by lens (26) produces a voltagepulse on the video output of the diode array giving indication where thespot is located on the array. The shape of this video pulse follows theGaussian power distribution of the laser; its magnitude depends on theintensity of the reflected light and the time delay between twosubsequent scans. To accommodate the changes in intensity of theilluminating beam and a secondary system which is based on generatingincreasing time delays between subsequent scans.

When a new reading is initiated by the computer or an external controldevice, the primary light level compensation is activated. The pocketscell controller takes a voltage measurement, of the video pulse througha peak hold circuit and based on this measurement, it either reduces orincreases the intensity of the illuminating beam. The pockets cellvoltage, and therefore the light output, is held constant for theduration of the upcoming dimensional measurement.

The dimensional measurement is initiated by the microcomputer (44) afterthe pockets cell has settled down to a new intensity level. The lineardiode arrays are operated at a constant clock rate put out by the lightlevel compensation board. The same board (PC 7808) also generates thearray start pulses.

The amplitude of the signal generated by the photo diode arrays isproportional to the light integration time for a given optical powerlevel. The light integration time is the elapsed time between the startof two subsequent scans. Thus, controlling the integration time allowsone to control the diode array output voltage: meanwhile the clock-ratecan be kept constant. The constant clock-rate permits the use ofconstant bandwidth video filters (41) as the frequency content of thevideo signals remains unchanged from scan to scan. These filers arerequired to overcome the problem of multiple triggering on speckleeffect induced high frequencies in the video signals.

The system used in this gage generates start pulses to the diode arraysby a geometric sequence of T=t, T=2 t, T=4 t, T=8 t etc, where t is theunit integration time. Thus the video output voltage varies as 1, 2, 4,8 etc. with the increasing integration time on each subsequent scan.Each integrating time period consists of two parts, namely a wait periodand a scan period. In this design, t equals the array scan time. Thus,during T=t, the wait period equals zero, in T=4 t, T=8 t, etc. the waitperiod is greater than the scan period.

The above binary sequence is generated on PC 7808 by the binary counterchain formed by D6, D5, D4, D11 and D10. The sequence number isgenerated by counter D9 and selected by multiplexer D12. At the end ofeach diode array scan D9 is incremented and a longer integration timegets selected by D12. When the wait period is over, a start pulse isgenerated, which initiates a diode array scan. The sequence isterminated, when either both video signals reach a fixed thresholdlevel, or a scan cycle finished signal is generated by comparator D15,which limits the binary sequences to a number preset by a thumbwheel.

The optical triangulation method utilized in this design derives thedimensional information about the part by computing the position of thecentroid of the light spots falling on the diode arrays. Rather thangoing through a complex computing method, a simple counting techniquewas devised to find the optical center or centroid of the spot as shownon FIG. 5.

Each diode array element is counted as 2 in zone A and when the videosignal goes over the threshold in zone B, each element is counted asone. When video falls below the threshold at the end of B all countingstops at the end of the diode array scan. The counter, therefore,contains the position information. The count at this point is twice thenumber of array elements between the starting point and the center lineof the spot, but by taking into account that the threshold is crossedtwice, the accuracy of this number is twice as high as if A+B/2 wascomputed. Taking this argument further, a multiplicity of thresholds canbe used to improve the position resolution to a greater degree. A systemwith 2 thresholds is shown in FIG. 6.

PC 7807 4 level Spot Centroid Finder has been extended to 4 thresholdlevels. Therefore, theoretically it will be a 4× improvement inaccuracy.

Video filter selection is provided by analog switches A1, A2 and decoderA3. The output of buffer amplifier A4 is fed to the Inverting inputs oflevel detectors A7, A8, A9, A10. Buffer amplifiers in A6 provide fixedthresholds to the just mentioned level detectors. The threshold levelsare produced by a resistor chain R3-10 and adjusted by potentiometerR12. Edge triggered flipflops D4, D5, D6, D11 detect the leading edge ofthe video pulse on their pin 11 inputs and the trailing edge on theirpin 3 inputs. The outputs of the flip-flops are interrogatedsequentially by data selector D2, selected by counter D1. The clockinput to D1 is eight times faster than the diode array clock rate. Theoutput of D1 is a count pulse sequence containing the positionalinformation on the optical centerline of the laser spot.

To avoid jitter in the readings, the video signal level has to reach thethreshold on All to validate the data. If this threshold is not reachedduring a scan, a new scan is initiated by PC 7808 after an integratingtime twice as great as the previous one.

In cases where higher speed operation is necessary, it is possible toprocess the video signal using a delay line technique as follows. Thevideo signal generated during a given scan is processed to give anaverage (or peak) power reading of the light spot intensity. During thissame time period, the signal is passed through a delay line. The average(or peak) power reading is then used to set the threshold levels used toobtain the spot center locations as discussed above.

An example of the adaptation of the optical gaging technique to aspecific problem is the measurement of the pitch circle runout of gears(46). Two optical triangulating systems and their electronics are usedto perform the measurement. One is used to form a reference location andtriggering unit and is referred to as the reference sensor (47). Theother is used to make the measurement on the command of the referencesensor and this is referred to as the measurement sensor (48).

Both units are the same, consisting of a laser light source (49, 54) anda pockel cell (50, 55) to control the light output intensity. The lightoutput is focussed to a small diameter spot by a focussing lens (51, 56)and an image of this light spot on the object is focussed by an imaginglens (52, 57) to form an imaged spot on the linear photo diode array(53, 58). The linear photo diode array is used to measure the positionof the light imaged spot which is then related to the position of thesurface of the gear with respect to the measurement unit.

The reference and measurement sensors (47, 48) are directed radiallytowards the centre of the gear (FIG. 8). The optical systems are alignedso that the measurement range or the sensors is centered on the pitchcircle radius of the gear. For a gear with an even number of teeth thesensors are diametrically opposite each other and for an odd number ofteeth the sensor has a rotational angular offset of half a gear tooth.

On the rotation of the gear (46) about its axis, the reference sensormeasures the radial position of the gear tooth surface with respect tothe pitch circle radius. When the required surface of the gear tooth isat the pitch circle radius as shown in FIG. 8, the reference-sensortriggers the measuring sensor to take a measurement of the radialposition of the surface of the gear teeth opposite the reference sensor.For a perfect gear this surface will also be at the pitch circle radius.However, if there is some pitch circle runout on the gear, which isformed by the geometric centre of the gear teeth being displaced fromthe rotational axis of the gear, the position of the surface beingmeasured will effectively move away from the pitch circle radius in acycle manner as shown in FIG. 9. The amount of radial displacement ofthe surface of the gear teeth as the gear is rotated through a completerevolution is equal to twice the displacement of the pitch circle centrewith respect to the rotating axis of the gear.

Additional parameters of the gear can be derived by some modificationsof this concept. Individual tooth to tooth spacing can be measured byobserving adjacent teeth with the time sensors as shown in FIG. 8. Theinvolute profile of the gear teeth can be measured with the addition ofa high resolution angular encoder mounted to the gear rotation mechanismand facilities for traversing the sensor head in the radial direction.These modifications would convert the whole system with a twodimensional measurement device operating in Polar coordinates. Movementof the measurement sensor in a direction parallel to the axis of thegear (46) would enable the three dimensional form of the gear teeth tobe measured to provide information on the helix angle of the gear andthe lead deviation.

The gear involute or helix angle can also be measured at high speed by afurther modification to the optical triangulating technique as shown inFIG. 10. In this case, an array of point measurements are madesimultaneously on the gear involute or helix angle form by a bank ofoptical triangulating sensors rather than moving the gear or thetriangulating sensor to produce the required number of measurements.Each individual sensor consists of a light source (63), focussing lens(64), imaging lens (65), and linear photo diode array detector (66). Toachieve a high speed of measurement, the light sources would be pulsedsimultaneously to freeze the motion of the component in the correctposition. In this case the light sources would be pulsed semi conductorlasers or similar devices. The information from the linear photo diodearrays is read by the system electronics and transferred to amicroprocessor for the final shape calculations.

This technique is also applicable to the rapid measurement of discretepoints and parameters using optical triangulation on other componentssuch as complete gears, turbine blades and other mechanical systems.

Shown in FIG. 11 is another example of the use of optical triangulationto a specific measurement is a system that is designed to measure thethickness of a small flat pump vane (67) at a high part rate. Thistechnique utilizes two optical triangulating sensors, operating in adifferential mode to accommodate the errors in the position of thecomponent as it slides down a track across the measurement zone.

The two triangulating sensors (68, 69) are identical and consist of alaser light source (70, 74) whose output is focussed down to a smalldiameter spot on the surface of the component by a focussing lens (71,75). An image of this light spot is transferred to a linear photo diodearray (72, 76) by an imaging lens (78, 77). The linear photo diode arrayis used to measure the geometric position of the imaged spot which isrelated to the position of the surface of the component with respect tothe triangulating sensor.

Each of the sensors is placed on opposite sides of the thickness regionof the component that is being measured. The component (67) istransported along a mechanical track across the measurement zone of thetwo triangulating sensors. The presence of the component in themeasurement zone is detected by the sensors and, as the component movesacross the zone, a number of measurements of the thickness are taken.The thickness is generated by measuring the position of the two surfaceswith reference to their respective sensors and subtracting these valuesto form the thickness.

The electronic technique for performing the measurement function isoutlined in block diagram form in FIG. 12. The video output of thelinear photo diode array for each of the two sensor units is processedin the same manner. The electronic system consists of the controlelectronics for the diode arrays (78, 79) a signal conditioning system(80), a microcomputer interface (81) and a microcomputer (82). The lightintensity of the laser (70) and scan speed of the linear photo diodearray (72) are adjusted to make the response of the linear photo diodearray approach saturation to produce well defined edges of the imagedspot on the video signal. The position of the edges of the image andposition of the total image is measured in the signal conditioning unit(80) by a thresholding technique as described in the section on theelectro-optical system for optical triangulation.

The positions of the images on the two linear photo diode arrays aretransferred from the signal conditioning electronics (80) to the microprocessor (82) via the microprocessor interface (81). Within themicroprocessor this information is translated into thicknessmeasurements. A series of thickness measurements of the component arestored and analyzed in the following manner. The most frequentlyoccurring thickness value for the part is noted. The measurement valueswithin a given band of this value are averaged to provide the finalmeasurement. Information outside this band is rejected. This processingtechnique is used to find a time mean value and to accommodate theeffects of randomly occurring surface finish and roughness variations.

Besides the variable integration time approach used to achieve operationin the above embodiments under widely varying light level, it is alsopossible to vary the clock rate of the scan. This is done by speeding upthe scan rate when an excess of light is present and slowing it downwhen less light is reflected from the surface into the optical system.

This variable clock rate system is effective but unfortunately resultsin a varying frequency output of the gage which makes it difficult toachieve the use of video filters which is effective for decreasing theeffects of speckle and other surface noise.

One advantage of the variable clock rate, however, is that the clockrate can be varied within an individual scan. This can have use whenmore than one image, for example, is on the surface as in the case ofprojecting two zones on the part. It also can have advantage when it isdesired to equalize even the power coming back from even individualsections of any spot.

Also of interest is the use of delay lines, either analog or digital atthe output of the diode arrays in order to store the previous diodearray trace and so determine from the stored value what the sensitivityselection in terms of integration time or clock rate should be for thenext scan.

It should also be noted that it is possible to use two axis rather thanone axis photo diode arrays in this invention. For example, use of asquare matrix photo diode array allows spot centers to be tracked evenif they move off in another plane and it allows their true spotcentroids in two planes to be calculated.

A two axis array is also of value when more than one spot in one axis isprojected as is discussed below in the context of sensing attitude.

Circular arrays may also be used. Of particular value would be a radialdiode array with all lines emanating from a center. Such an array is nottypically available at present but it is thought that it will be in thefuture and use of such an array allows polar coordinate symmetry to beutilized.

Other Zones

In the foregoing there has been discussed the projection of a spot typezone onto the part (e.g. 2A in FIG. 1) and the corresponding readout ofwhere the spot position is. Shown in FIG. 13, however, is a projectionof a line onto the part which is surrounded by two light levels. Ineffect the zone either shaped like two dots with a dark zone in betweenor four dots with two lines in between, almost reticle fashion. Thediode array unit described above, utilized in a manner similar to FIG.1, can easily find the center of the dark zone between the two brightzones and from the metrological point of view, this is perhaps a bettertype of measurement than measuring the centroid of a single spot.

The question, of course, is how to provide the sharply defined line onthe surface? One way is to simply image a line or cross hair on thepart, which is achievable as long as the distance variation is not toogreat from the focal point. Naturally, as one gets farther away from thefocal point of the projection system, the projected grid line wouldbecome fuzzier and correspondingly harder to accurately define. The lineshould be very fine in order to create the minimum possible analogdefinition required of its location, and this requires close focus.

A second possibility is to project the TEM₀₁ mode of the laser which isin fact comprised of just such a pair of spaced spots. In this case theprojection system would be very similar to that shown in FIG. 1. Theonly difference is that the TEM₀₁ mode as a whole is broader but theline between is smaller than the corresponding TEM₀₀ spot. It is notedthat when a two axis grid is produced, the matrix diode array discussedabove is utilized to sense its position in two axes. This can also giveobject surface attitude information (see FIG. 14).

Correspondingly, more numbers of lines such as 2, 3, 4 etc. can beutilized. If this is carried to an extreme, one looses the ability totell the absolute value of where the center is. For this reason it isthought that in the system described here, one or two lines is mostuseful. If more lines are used they should be perhaps be of differentthicknesses etc. so that one knows which is which.

Another embodiment of the invention is shown in FIG. 14. In this case, atwo axis matrix diode array camera 200 is added to the FIG. 1 typesensor 201 to view the area immediately around that of the measuringpoint 202A, using beam splitter 203. In this case, several advantagesare obtained. First, a video display 204 can be provided which allowsthe operator to see the zone in which the measurement is taking place.

Second, the immediate surroundings of the part can be measured in thex-y plane simply from the matrix diode array image and as such it maynot be necessary to go to all of those points in successionelectro-mechanically in order to perform a measurement of, for example,a hole diameter. This is shown in the example wherein an automotivedashboard 205 is being measured with the back lighting to illuminateholes such as hole 206. The contours around the hole and of the shape ofthe part are measured as in the FIG. 1 case where the hole measurementbeing taken from the matrix array which is linked to a computer todetermine the diameter and out of round needed of the hole.

A third advantage in utilizing this bore sited camera is that one isable to sense ahead of the measuring point and to in essence, tell theunit where to go. In other words, if it is seen through the matrix arraythat an edge of a part such as edge 220 is being approached, this edgelocation can be noted in the program and the sensor unit told to move toanother location. In other words, considerable time savings result.

A final embodiment of the invention describes the sensing of attitudeusing the invention and its further incorporation into control systemsfor robots and other automation.

In the embodiment of FIG. 14, a three-axis capability has been provided,one axis (z) with the triangulated measuring point, and the two axes(x,y) with the matrix diode array operating either in transmission orreflection from the part object. It is now desirable to add a furthertwo axes of sensing to make a five axis sensor unit. The two axes inquestion are yaw and pitch or the angles θ and φ as seen from thesensor. 0 lies in the xy plane, θ in the y-z plane.

There are two ways of adding this sensing. The first, as shown in FIG.15, is simply to project two beams instead of one and view the image ofbowel spots on the linear photo diode array (or diode arrays if two areutilized, one on each side as in FIG. 1). In this case, the spacing ofthe spots is proportional to the angular tilt of the surface whereas theposition of the center of the two spots on the array is proportional tothe distance of the surface from the sensor head. Sensing of the twospot separations gives the angle of the surface in the plane ofmeasurement.

If two sensors, one on each side in the same plane are utilized as inFIG. 1, an interesting feature results in that when the part isperfectly normal to the sensor head, both of the units, one on eachside, are at equal angles from the center beam and read identically.This can increase the accuracy obtainable from the angle measurementconsiderably and also gives a very clear definition of normalityindependent of other features.

Naturally, if attitude measurement in two planes, θ and φ is to beaccomplished, sensing is required in the two axes x and y to the first.This situation is shown in FIG. 16 which shows a 5 axis sensor headembodiment of the invention, 300, mounted on a robot arm, 301, andsensing in θ, φ, x, y, and z. This sensor is completely capable ofcharacterizing the total location of a part 303 near a robot for thepurpose of either inspecting the part or of performing some manipulationon it automatically. The sensor head axes are all fed back to the robotcontrol computer which then calculates the best approach and continuallyupdates this calculation with continued sensor data until the robot endeffector ("hand") has grabbed the part or done whatever operation isrequired.

It is clear that by combining sensors of FIGS. 14 and 15 such a 5 axiscapability can be provided. However, a particularly compact and simpleunit is shown in FIG. 16 wherein u matrix array camera 304 views thepart in x and y, using for example flashed Xenon ring light source 306.A four spot projector 309 (see also FIG. 13) comprise a nest of fourdiode lasers 310 imaged onto the part by lens 312. A micro computer isutilized to interpret the data from array 305. First, the Xenon lamp isflashed, and the x and y location of a feature 320 of interest on part303 is determined. Then the laser diodes are flashed, and the positionof the 4 spot images is determined from the matrix array output.Analysis of the location of the center of the spot group gives range,while the individual spot spacings, S₁ and S₂ give θ and φ.

Now described are particular circuit embodiments primarily useful onsensing moving parts, for example rotating gear teeth, etc. Changes inreflectivity of the part being measured can cause errors in finding thelocation of the spot centroid, due to erroneous level detection. Themethod described earlier, which is based on the external control of thelight integration time combined with a pocket cell and its controller issuitable for measuring quasi-stationary objects only.

The only method can be used in optical triangulation systems, wheneither the object being measured is in motion, or the light source is ofa pulsed type or a combination of the two. The problem, when the partbeing measured is in motion, is that the local reflectivity of the partis in constant change as the surface moves. Variations can be caused byangular changes or can be simply the local variations in color andreflectivity. The same applies when the light source used is of a pulsedtype (such as pulsed lasers, flash tubes etc.), with the additionalproblem that the optical power of the returned beam only becomes knownafter the pulse has occurred.

To overcome the above stated problems, one can use a separate detectorsystem to look at the reflected pulse and turn off the light source whena given accumulated energy level is reached. This is commonly done inthyristor flash guns used in photography for example.

An attractive approach is shown in FIG. 17. The signal coming from thediode array, 500, is filtered by video filter 501, and is fed to thevideo delay line, which is capable of storing one complete video stall.The signal going into the video delay line is peak height detected bypeak hold, 502, and the detected voltage is transferred-to the sampleand hold circuit '503, at the end of the scan. Once the complete videoscan is stored, the sample and hold amplifier sets the level detectorthreshold and the stored video is clocked out of the delay line and tothe comparator, 504, which may operate with a multiplicity ofthresholds. The threshold detected video in conjunction with thecentroid finding circuit (described elsewhere in this disclosure)produces a series of count pulses to be summed in a counter.

An advantage of the above system is that the threshold for thecomparator can be set after reading out the diode array. Therefore, thecompensation for greatly varying video signals can be made automatic.

Another version is shown in FIG. 18. The significant difference is thatthis version has a variable gain amplifier, 550, whose gain iscontrolled in inverse proportion with the area under the curve,determined by fast integrator, 551, which represents the optical energyin the detected laser spot.

The operating sequence of this version is the same as that of FIG. 17,namely the filtered video signal is stored in the video delay system forthe duration of one scan period, at the end of which the contents of thefast integrator is transferred and is then held in the sample and hold,circuit 552 for the duration of the next scan. Constant output voltagelevel is maintained by setting the gain of the variable gain amplifierin inverse proportion to the detected video levels. The output signal ofthe variable gain amplifier is compared to a fixed voltage or amultiplicity of voltages through a number of comparators to producesignals for the centroid determining circuit which in turn finds theposition of the spot centroid.

It is of interest to conclude this application with a further discussionof five axis robot guidance (e.g. FIG. 16) plus a code system for simpleorientation of parts.

The robot guidance head has been shown in FIG. 16. This is a combinationof two different sensor technologies namely, triangulation to determinerange and attitude in two planes (pitch and yaw), and two axis imagescanning to obtain various features in the xy planes. The invention hereprovides a combination of these two technologies of particularsuitability for the guidance of robots. If, for example, one considersthe placement of such a five axis sensor. (x, y, z, θ and φ) on or nearthe end effector ("hand") after the last joint of a robot arm or othermanipulator portion, this sensor then is fully capable of telling atleast within its field of view exactly what the situation is relative tothe sensor hand. This is then totally like the human in that it can tellthe general characteristics needed to orient the hand to the part.

The simplest version is shown in FIG. 16 wherein the z axis range isobtained through triangulation, and wherein double range differentialrange measurements in each axis provide the attitude. The x and y visionis provided by conventional imaging systems, in this case a matrix photodiode array.

In the particular version shown the xy portion is provided using a lenstogether with the square matrix diode array system. Light is provided bya ring flash light source located around the periphery of the cameralens. Other light sources can be used, but this is the most convenientsince it is flashed which keeps the camera from blooming and alsobecause it is compact and provides relatively shadowless illumination.Actually shadows may be desired in some applications to providecontrast, but for generalized movement of the arm, it is considered thatshadows would actually be a detriment since one would not know the angleof attack in every situation to the particular portion of the part inquestion.

Arrayed around this central portion are four CW, or preferably, pulsedcollimated light sources. These project spots of light onto the part ata given distance and can be pulsed in sequence. The light sources maycomprise light emitting diodes or diode lasers, but it is possible toform spots by breaking up a single beam into spots with a grating.Pulsing of light sources in sequence of importance because the operationof the system is to do just that in ripple fashion such that at any onetime only the image of the particular spot in question exists. This wayone can tell which image is on the diode array. It is not necessary inall cases but it is of considerable use.

An adjunct or alternative application of robot guidance is in the codingof the parts to be robotically manufactured. Normally, such codes areput on to serialize the parts for identification as to lot number etc.as well as serial. This, of course, can be done here as well withautomatic bar code reading etc. (The matrix or linear arrays heredisclosed prove excellent for this).

However, one of the more interesting aspects is that such a code canalso be an orientation code, put on for the purposes of providing aneasy reference for the two axis matrix diode array of the robot arm fromwhich the orientation of the part may be determined. Naturally, this isof particular interest on those parts that have complex features thatmight make it difficult to otherwise orient.

The use of an orientation code, if possible, therefore essentiallysolves two of the vision requirements of the robot to a far more trivialstate than would otherwise be the case. With such codes, no longer doesthe computer program in the robot have to identify which part is which,nor determine the possible complex orientation required to pick the partup or place it into a machine.

For example, again consider FIG. 16 in which case an orientation codesticker 700 is provided on the part together with a separate bar codepart type representation, 710. While these two codes could be combined,it is thought here to be a better idea, at least at first, to separatethem out.

The orientation code contains a set of intersecting axial lines whichserve to determine the xy orientation. Also on this code is a sub-codewhich gives the third dimensional orientation, for example, in degreesof tilt.

Another way of giving third axis information is to look up in thecomputer using the part type bar code representing the orientation ofthe part when lying on a conveyer. A combination approach using theapparatus of FIG. 16 is to have the robot arm come down until the sensoris a fixed distance from the part (which can be obtained with a simpletriangulating sensor on the arm). The unit is then programmed so thatthe code when viewed at the correct angle will measure the code bars tothe spacing known to exist in normal incidence viewing.

In this case the pitch and yaw sensors would not necessarily be requiredhere.

It is noted that the word robot also can be characterized by the worduniversal transfer device (UTD). However, the concepts here disclosedare apropos to control all sorts of material handling automation and notnecessarily those of a universal nature.

What is claimed is:
 1. Apparatus for positioning a member in a desiredposition relative to a surface of an object comprising:positioning meansfor positioning a member, non-contact sensing means to sense theattitude of said object surface relative to one of said member and saidpositioning means, said attitude being sensed in at least one planenormal to said object surface, and control means to control saidpositioning means using said data from said sensing means to positionsaid member in a desired attitude relative to the surface of saidobject.
 2. Apparatus as in claim 1, wherein said non-contact sensingmeans comprises an electro-optical sensor.
 3. Apparatus as in claim 1,wherein said electro optical sensing means uses triangulation to sensesaid attitude of said object surface.
 4. Apparatus as in claim 3,further comprising means for projecting a plurality of zones of lightonto said object surface, said electro-optical sensor sensing lightreflected from said object surface.
 5. Apparatus as in claim 1, whereinsaid non-contact sensing means is adapted for sensing said attitude intwo planes, each of which is normal to said object surface.
 6. Apparatusas in claim 5, wherein said control means is adapted to control saidpositioning means to position said member at a desired attitude relativeto said surface of said object in two planes, each of which is normal tosaid object surface.
 7. Apparatus as in claim 1, wherein said controlmeans is adapted to control said positioning means to position saidmember at a desired attitude relative to said surface of said object ina plane which is normal to said object surface.
 8. A method forpositioning a member in a desired position relative to a surface of anobject comprising the steps of:providing positioning means forpositioning a member, sensing the attitude of the object surfacerelative to one of said member and said positioning means using anon-contact sensing means, said attitude being sensed in at least oneplane normal to said object surface, said sensing means providing datarelated to said attitude of said object surface, and controlling saidpositioning means using said data from said sensing means to positionsaid member at a desired attitude relative to the surface of saidobject.
 9. A method according to claim 8 wherein said positioning meanspositions said member normal to the surface of said object.
 10. A methodaccording to claim 8 wherein said member is part of said positioningmeans.
 11. A method according to claim 8 wherein said member is graspedby or other wise attached to said positioning means.
 12. A methodaccording to claim 8 wherein said positioning apparatus is operable in 4or more axes.
 13. A method according to claim 12 where said apparatus isa robot.
 14. A method according to claim 8, wherein said sensor, or anadditional sensor, inspects said object after said positioning isaccomplished.
 15. A method according to claim 8, further comprisingcalculating a desired approach for positioning said member relative tosaid object based upon said data provided by said sensing means.
 16. Amethod as in claim 8, wherein said sensor means comprises anelectro-optical sensor, wherein said attitude is sensedelectro-optically.
 17. A method as in claim 16, wherein saidelectro-optical sensor uses triangulation to sense the attitude of theobject surface.
 18. A method as in claim 17, further comprisingprojecting a plurality of zones of light onto said object surface andsensing light reflected from said object surface.
 19. A method as inclaim 8, wherein said attitude is sensed in two planes, each of which isnormal to said object surface.
 20. A method as in claim 19, wherein saidcontrolling step comprises controlling said positioning means toposition said member at a desired attitude relative to said surface ofsaid object in two planes, each of which is normal to said objectsurface.
 21. A method as in claim 8, wherein said controlling stepcomprises controlling said positioning means to position said member ata desired attitude relative to said surface of said object in a planewhich is normal to said object surface.